A regulator-ballast circuit for HID lamps is disclosed in U.S. Pat. No. 4,891,562. In that circuit is used a magnetic regulator having three windings, one winding being connected to a lamp, another to an AC source and the third to a control circuit. The patent discloses a control technique using a switch, the firing angle of which is controlled to regulate the operating characteristics of the lamp, despite line voltage changes, lamp aging effects and other variations. It is also pointed out therein that the control circuit can be used as a lamp dimming circuit.
There are a number of circumstances in which lamp dimming is desirable and others in which power conservation is the main objective and lamp dimming is acceptable. Consider, for example, a space which is illuminated by one or more HID lamps and which is occupied sporadically. In order to save energy, it would be possible to sense that the space is unoccupied and simply deenergize the lamps completely. However, with HID lamps, restarting usually takes some time and can be difficult under some operating conditions, as well as tending to reduce lamp life. Also, in many circumstances, it would not be desirable to entirely eliminate illumination in the absence of an occupant for security or other reasons.
The dimming circuit disclosed in the above patent (FIG. 2) is capable of providing smooth dimming over a wide operating range. While that is certainly desirable for many reasons, the circuit uses a large number of components and is therefore costly. For those situations which call for reduced power usage and in which this smooth dimming is not essential, it would be preferable to employ a step system which uses fewer components, thereby reducing cost, increasing reliability and increasing the ruggedness of the apparatus.